1. Field of the Invention
The invention relates generally to apparatus and methods for stabilizing the wavelength of light sources, and especially to the use of a temperature-compensated reference interferometer with one electro-optic crystal and two passive birefringent crystals to effect such stabilization.
2. Description of the Related Art
The phase velocity of a light wave having a given wavelength and traveling through a material medium is inversely proportional to the index of refraction of the medium at that wavelength. For certain materials the index of refraction depends on the polarization of a light wave propagating through them. Such materials are said to be "birefringent."
The property of birefringence can be distinguished as either passive or active. In passive birefringent crystals the birefringence is constant and cannot be varied by an external field, whereas in active birefringent crystals the application of an external field, such as an electric or magnetic field, can vary the birefringence in them. Examples of passively birefringent materials include monocrystalline silicon dioxide and magnesium fluoride. Examples of actively birefringent materials include monocrystalline lithium niobate and lithium tantalate; their birefringence can be varied by the application of an electric field.
A linearly polarized light wave can be thought of consisting of two component waves having polarization vectors at right angles to each other in some reference coordinate plane. In traveling through a birefringent medium the two component waves will travel at different speeds because they encounter different refractive indices due to their different polarizations. After having propagated through a given path length the phases of the two component waves will be different because of their different speeds. If the phase difference is one-quarter of 2.pi. radians, i.e., .pi./2 radians, the slab of material making up the path length is referred to as a "quarter-wave" plate.
Several configurations have been proposed for stabilizing the wavelength of superluminescent diodes (SLDs) utilized as the optical source in various applications, such as in fiberoptic gyroscopes. One of these configurations, based on two passive birefringent crystals (silicon dioxide and magnesium fluoride) for the reference interferometer and on dc detection, is disclosed in U.S. patent application Ser. No. 017,426 to Hall, filed Feb. 20, 1987, and now U.S. Pat. No. 4,842,358 and assigned to the assignee of the present invention. The disclosed method of stabilization suffers from the disadvantages of poor signal-to-noise ratio and low sensitivity due to dc detection. Another configuration is disclosed in U.S. patent application Ser. No. 017,425 to Wilson, filed Feb. 20, 1987, and now U.S. Pat. No. 4,890,922 and assigned to the assignee of the present invention. In the Wilson application two electro-optic crystals are used (lithium niobate and lithium tantalate) with a modulating signal applied to the crystals through electrodes for synchronous (ac) detection. This configuration has the disadvantage of requiring large modulating voltages as well as stringent fabrication tolerances. In synchronous detection a detection system is used which is sensitive only to signals at or near a given frequency or one of its harmonics, the frequency being the same as a modulating frequency applied to the electrodes of the active crystal. Synchronous detection obviates various noise and slow electronic drift problems. The passive pair of crystals is designed for temperature compensation and acts as the reference interferometer. Compensation of the reference interferometer for shifts in temperature insures that the phase differences being measured by the interferometer are due only to shifts in the wavelength of the optical source. This configuration of electro-optic and passively birefringent crystal pairs suffers from the disadvantages of requiring four crystals, with the two electro-optical crystals being necessarily of exactly the same length, which as a practical matter is extremely difficult to achieve. In addition, out of the four crystals only the two passive crystals are stabilized with temperature, so that the overall performance can be severely degraded if the electro-optic crystals are not exactly the same length.
The three patent applications referred to above are hereby incorporated by reference into the present application.